The present invention relates to an apparatus for controlling a hydraulically operated continuously variable automotive transmission including a hydraulic actuator operatively coupled to a transmission ratio controlling member for continuously operating the transmission ratio controlling member to increase or reduce the transmission ratio of the continuously variable automotive transmission, and, in particular, to such control apparatus having a control valve for applying a controlling hydraulic pressure to the hydraulic actuator dependent on the difference between a first control force commensurate with an indication of the driver's intention of acceleration or deceleration and a second force commensurate with a rotational engine speed, in order to change the direction in which the hydraulic actuator is operated.
Conventional apparatus for controlling hydraulically operated continuously variable automotive transmissions have a control valve including a cylinder and a spool valve slidably disposed in the cylinder, the spool valve being movable dependent on the difference between first and second control forces applied to the control valve. The control valve is connected to a hydraulic actuator that is operatively coupled to a transmission ratio controlling member of the hydraulically operated continuously variable automotive transmission.
In such a conventional control apparatus, the rate of change di/dt of the transmission ratio i per unit time can be given by the equation (1): ##EQU1## where Neo is a target rotational engine speed, Ne is a detected rotational engine speed, and K is a gain. The detected rotational engine speed Ne is expressed by the following equation (2): EQU Ne=C.multidot.V.multidot.i (2)
where V is the speed of travel of the automobile or the output rotational ending speed, and C is a constant. Therefore, the rate of change dNe/dt of the detected rotational engine speed Ne is as follows: ##EQU2## Since the automoble speed V does not immediately vary greatly even if the speed is changed while the automobile is running, dV/dt.apprxeq.0, and hence the equation (3) approximates to the following equation (4): ##EQU3## From the equations (1) and (4), we get ##EQU4## where K'=C.multidot.K.
Since the gain K is constant and so is the gain K', it can be understood from the equation (5) that dNe/dt is proportional to the vehicle speed V.
If the gain K' in the equation (5) were set such that dNe/dt would be of an appropriate value at a low vehicle speed V, then dNe/t might be too high when the vehicle speed V would be increased. This would cause the engine rotational speed to increase or decrease abruptly, thereby allowing the difference (Neo-Ne) to go alternately positive and negative in repeated cycles, with the result that the operation to change the transmission ratio would become unstable.
Conversely, if the gain K' were selected such that dNe/dt would be of an appropriate value at a high vehicle speed V, then dNe/t might be too low when the vehicle speed V would be reduced, resulting in a slow response.
The above problem can be solved by reducing the gain K' at a higher vehicle speed and increasing the gain K' at a lower vehicle speed. This can be achieved by varying the amount of controlling hydraulic pressure supplied to the hydraulic actuator according to the vehicle speed for thus changing the speed of operation of the hydraulic actuator according to the vehicle speed.
One simple means for varying the amount of controlling hydraulic pressure supplied to the hydraulic actuator would be a variable restriction disposed between the hydraulic actuator and the control valve that has an intermediate position in which the degree of restriction is continuously variable for directional control. With the variable restriction, however, insofar as the stroke of the spool of the control valve is small, characteristic curves, as shown in FIG. 4 of the accompanying drawings, indicative of the amount of controlling hydraulic pressure supplied to the hydraulic actuator would not reflect different vehicle speed parameters .alpha.1', .alpha.2', .alpha.3', .alpha.4', . . . which determine the degree of opening of the variable restriction, and fine transmission ratio control would be impossible to perform.